Peculiar plasma ball

The green color of this plasma ball raises an important question: what gas fills this bulb? Of all the noble gases, none can provide with such color (helium=pink-yellow, neon=red-orange, argon=reddish-purple, krypton=whittish, xenon=blueish-white), and low-pressure mercury vapor provides with a blue discharge. Could it be a molecular gas? a volatile compound of barium or thallium? nobody, at the museum where this interesting plasma device is diplayed, knows about its specs.
By : MaxFG